“Ohhh, so now pot’s a ‘medicine,’ right? Now they’re handing out prescriptions? You gotta be kidding me!”

I’m in a taxi, being driven by a short-tempered man around 50 who's wrestling the vehicle through New York traffic with the finesse of a matador and the impassivity of the dictator of a small country.

When I climbed in at Penn Station, I made the mistake of telling the driver that I was going to meet a researcher who studies medical marijuana. The result has been a twenty-minute tirade that has been at times vituperative, dismissive, and scathing.

But it’s been difficult to figure out what his stance on marijuana is. He’s not against it per se. It’s more that he thinks the whole idea is a big joke.

As I’m listening to this rant, keeping an eye out for the bumpers of other cars, I’m having two reactions more or less simultaneously.

First, what I’m hearing sounds eerily familiar. The opinions that he's spouting are pretty much what I was saying a year ago when I first started writing my new book, Stoned: A Doctor’s Case for Medical Marijuana. In fact, I’m pretty sure that if he and I had met a year ago, we would have shared a good laugh about the grand joke of “medical” marijuana.

But it’s my second reaction that surprises me: I find myself disagreeing. With a New York City cab driver. I should state for the record that this is not something that I have ever been inspired to do before.

In fact, I'm now pretty certain marijuana really does offer some medical benefits.

In the past year, I’ve talked with dozens of experts, patients, and activists. I’ve slogged through hundreds of studies of marijuana. I can even say that I’ve had some limited experience with its effects on me. Along the way, I’ve graduated from a hard-nosed skepticism to an open-mindedness that I never would have believed possible a year ago.

So: Does marijuana work? Well, one thing I’ve learned is that this isn’t the right question to ask.

Does a hammer work? Sure, if you want to pound a nail. But if you want to fix a frozen iPhone, not so much. So whether marijuana “works” depends on what you want it to do.

When Marijuana Might Work ... And When It Probably Doesn't

Let's look at one example: Pain. Marijuana does seem to be quite effective in treating neuropathic pain, or pain caused by nerve damage.

There have been several clinical studies that used smoked marijuana (e.g. in a joint), with a placebo control. (Yes, there is ‘placebo’ marijuana, from which the active ingredients — cannabinoids — have been extracted using much the same process that is used to remove the caffeine from coffee beans). Those studies have found that marijuana can be very effective in reducing neuropathic pain, and probably also in increasing ability to function.

For instance, one study of 39 patients tested high-dose marijuana at 7% tetrahydrocannabinol, low dose marijuana (3.5%) and placebo. Patients were randomized and blinded to which version they were getting. That study found significant decreases in pain for both the high- and low-dose marijuana, compared to the placebo.

That’s neuropathic pain, but what about regular "nociceptive" pain? That’s more run-of-the-mill pain that’s caused by injury to bone or tissue. This is the kind of pain that you’d have after a hard workout, for instance, or pain due to arthritis or a broken bone. Could marijuana be effective for nociceptive pain, too?

Maybe, but the evidence is not nearly as strong, even in highly-controlled laboratory studies. In these sorts of studies, you expose volunteers’ skin to a piece of metal that’s been heated to a temperature that most of us would agree is uncomfortable (about 113 degrees). That’s their “pain threshold.” Then you see whether a drug lets people tolerate a higher temperature without squirming. In one such study, marijuana doesn’t seem to increase pain thresholds as much as some other drugs, such as morphine.

As long as we think of marijuana as an herbal remedy, I’ve become a pretty strong believer.

Also, there just haven’t been many good studies — mostly because there are lots of other treatments for nociceptive pain that we know are effective, like acetaminophen, nonsteroidals, and opioids (for severe pain). In other words, there hasn’t been a lot of incentive to study marijuana, when there are many other effective drugs already available.

So does marijuana "work"?

If you have neuropathic pain, it might. At least, it may be worth a try. But if you have regular, nociceptive pain, the jury is still out.

Where does that leave us? Is marijuana a "medicine"? A year ago, I found the idea as laughable as that cab driver does. It seemed as wishful and deluded as declaring single malt Scotch a medicine.

And I have to say I am still skeptical. Marijuana’s ingredients aren’t as refined and defined as the ingredients in a pill are. A joint is hardly a medicine.

So I think of marijuana as more or less equivalent to an herbal remedy. It’s essentially plant-based stuff with numerous active and inactive ingredients, only some of which we understand.

That’s not a fatal flaw. In fact, there are plenty of herbal remedies that are widely used, and even recommended by doctors. So as long as we think of marijuana as an herbal remedy, I’ve become a pretty strong believer.

Sure, we need more research — on its benefits, and also its risks. But I think we know enough now to be able to conclude that for some people it might be a reasonable treatment.

Adapted from STONED: A Doctor’s Case for Medical Marijuana by David Casarett, M.D., with permission of Current, a member of Penguin Group (USA) LLC, A Penguin Random House Company. Copyright (c) David Casarett, 2015.